It has long been known that glass substrates are weakened upon exposure to water. For example, moisture in air can cause weakening and the eventual breakage of glass.
Typically, glass substrates have been coated with polymeric compositions for various reasons, such as to protect and preserve the strength of a glass object, to prevent damage during handling, and to prevent moisture from attacking the glass substrate. In addition, coatings have been applied to optical fibers to decrease the microbending of the optical fiber, which can reduce the transmission of electromagnetic radiation through the fiber. However, it has been found that many coating compositions are water permeable, and therefore not very effective in protecting the glass substrate from moisture.
In addition to causing the weakening of glass substrates, moisture can also cause polymeric compositions that have been applied to the glass substrate to break away, i.e., delaminate, from the glass surface. The delamination of a polymeric composition can result in a weakened glass substrate, as the polymeric coating no longer protects the glass from environmental stresses.
Various coating compositions have been used to prevent the deterioration of glass substrates, and specifically optical fibers, to improve the useful life of the glass substrate to which it is applied. For example, U.S. Pat. No. 5,000,541, issued to DiMarcello et al., teaches a method for hermetically sealing an optical fiber with carbon, which prevents water from contacting the optical fiber, and thus prolongs the useful life of the optical fiber. Similarly, U.S. Pat. No. 4,849,462, issued to Bishop et al., teaches the incorporation of various organofunctional silanes into a coating composition to improve the adhesion between a coating composition for an optical fiber and the optical fiber, particularly in moist environments.
Likewise, U.S. Pat. No. 5,214,734, issued to Inniss et al., teaches the incorporation of particulate silica in a polymeric coating composition to increase the fatigue resistance of an optical fiber or glass to moisture.
Similarly, U.S. Pat. No. 5,136,679, issued to Broer et al., shows an inner primary coating that comprises an acidic phosphorus compound as an adhesion promotor. However, because it is hypothesized in the Broer et al. patent that an acid medium develops near the interface of the glass fiber and the first (inner primary) coating composition due to the presence of the phosphorus compound, this patent teaches away from the use of acidic phosphorus compounds in an outer primary coating or any other coating layer that does not directly contact the optical glass fiber.
Many coating compositions have drawbacks that make them unsuitable for certain applications. For example, some of the compositions may be too expensive to use in the production of low cost optical fibers or glass objects, and the introduction of particulate matter into a coating composition as disclosed in the Inniss et al. patent can present problems such as scratching of the pristine optical glass fiber, resulting in breakage at low tensile loads; a turbid coating composition which has a tendency to gel; and other processing problems that are commonly encountered when working with particulate matter.
Coatings which are modified with organofunctional silanes, such as the above-referenced Bishop et al. patent, do improve the wet adhesion of a coating applied to an optical fiber substrate, but do not significantly improve the wet adhesion retention capability of the coating, when compared to this invention.